1. Field of the Invention
This invention relates to a process for removing relatively small quantities of sulfur dioxide gas from gaseous mixtures.
2. Brief Description of the Prior Art
In my U.S. Pat. No. 3,984,529, I review various prior efforts to provide economical and efficeint processes for removing sulfur dioxide gas fromgaseous mixtures, such as stack gases or the like. Removal of sulfur dioxide is desirable in order to obviate or reduce atmospheric pollution, and also in order to avoid the loss of the economic value represented by the recovered sulfur or sulfur compounds constituting potential end products of the recovery process.
In my referenced patent, I describe procedures and certain proposed methods which generally include the step of initially contacting the gaseous mixture which contains the sulfur dioxide with an aqueous solution containing a removal reactant so as to form a condensed phase in which the removed sulfur dioxide is combined chemically with the removal reactant in the form of sulfite or hydrosulfite compounds in aqueous solution. The removal reactants contemplated for use in the processes described include, inter alia, alkali metal hydroxides, and salts of weak acids and alkali metal hydroxides.
After formation of the pregnant scrubbing solution which contains compounds including combined sulfite ions derived from the extracted SO.sub.2 gas, the pregnant scrubbing solution is contacted with an organic liquid phase which includes, as an active component, certain nitrogen-containing water-immiscible organic compounds which have specified properties. Among such compounds which are effective and are preferred at this point in the process are various types of long chain alkyl amines which contain from about 12 to about 45 carbon atoms, and have a solubility in water of less than 0.2 gm/100 gms of water at 25.degree. C., and the water immiscible salts of these basic compounds. When the amine salts are used as the transfer reactants, their water solubility should not exceed about 5 gms/100 gms of water at 25.degree. C. Such amine salts are generally derivable from free amines having the degree of immiscibility prescribed above. The result of contacting the pregnant scrubbing solution with the described amine-containing organic liquid phase is to transfer the sulfite ions from the pregnant aqueous scrubbing solution to the organic phase as a result of chemical combination of sulfite ions with the long chain alkyl amines. A concomitant result of the described contact is the regeneration of the scrubbing solution containing the selected removal reactant so that it can be recycled to the scrubbing zone, and used for contacting the gaseous mixture from which additional SO.sub.2 is to be subsequently removed.
The organic phase developed upon contact of the long chain amine with the pregnant scrubbing solution contains long chain amine sulfites, and is contacted with hydrogen sulfide so as to reduce the sulfur-containing salts therein to elemental sulfur, and concurrently regenerate the long chain amine. The amine can then be reutilized for contacting additional sulfite-containing pregnant scrubbing solution from the scrubbing zone for the purpose of repeating the transfer of sulfite ions into the organic phase.
When an amine salt is used instead of an amine as the transfer agent, the transfer step will yield an acid in the aqueous phase. This acid can be removed by contacting this aqueous phase with the organic phase from the reduction step where the regenerated amine will combine with the acid to form the amine salt. After this "recontact," the aqueous solution can be recycled to the scrubber and the organic phase can be utilized as recycled transfer agent.
My U.S. Pat. No. 4,101,643 described an improvement on the foregoing described basic process which comprises initially contacting the gas from which the sulfur dioxide is to be removed with an aqueous solution of an alkali metal sulfite. In the course of this scrubbing step, the SO.sub.2 in the gaseous mixture combines with the alkali metal sulfite to yield the hydrosulfite salt in aqueous solution. The pregnant aqueous solution resulting from scrubbing is next contacted with a liquid organic phase which contains one or more substantially water-immiscible long chain amine compounds capable of extracting sulfite ions from the pregnant scrubbing solution by chemical combination therewith. Upon the termination of intimate contact between the organic extractant phase and the aqueous phase, the alkali metal sulfite is regenerated in the aqueous phase so that the scrubbing solution thus formed can be recycled to the scrubbing zone of the process. The organic phase, which contains long chain amine sulfites upon completion of the transfer reaction, is then heated to a temperature sufficiently high that the amine sulfites therein undergo thermal decomposition to yield sulfur dioxide. The decomposition of the amine sulfites concurrently produces free amines which can be reused in carrying out further extraction of sulfite ions from additional scrubbing solution.
The sulfur dioxide generated by decomposition of the amine sulfite in the organic phase can be used as an end product, or it can be converted to element sulfur by various conventional methods known to the art.
In my U.S. application Ser. No. 838,912, now abandoned, it was proposed to use a catalytic amount of water inthe decomposition step. The presence of water in such amount facilitates the decomposition of amine sulfite and/or amine hydrosulfite to free amine and gaseous SO.sub.2.
In my cop-pending U.S. application Ser. No. 838,913, the SO.sub.2 is first contacted in a scrubbing zone with a scrubbing agent which is an aqueous solution of an alkali metal salt of a weak acid which has an ionization constant (pKa) value of at least 3.5. The pregnant scrubbing solution is then contacted with an organic transfer reactant which is a water-immiscible, water insoluble amine salt to effect a transfer reaction by which the sulfite ions are transferred to the non-aqueous phase by way of an ion exchange reaction. The sulfur bearing, non-aqueous phase is next heated to decompose the sulfite, yielding SO.sub.2 and a water-immisicible, water insoluble amine-bearing compound. The amine-bearing compound is next contacted with the aqueous solution from the transfer step. After this contact, the regenerated aqueous scrubbing solution is recycled to the scrubber, and the regenerated organic solution is recycled to ion exchange with additional pregnant scrubbing solution.
In Wiewiorowski U.S. Pat. No. 3,633,339, a process is prescribed for removing sulfur dioxide from gases, and initially entails contacting the gas containing the sulfur dioxide with an aqueous solution of ammonium phosphate. The pH of the thus constituted scrubbing solution is indicated by the patentee to be from about 3.0 to about 5.0 with the best range being between 3.5 and 4.5. The pregnant scrubbing solution contains ammonium hydrosulfite and ammonium phosphate. This pregnant scrubbing solution is then contacted with an organic extractant to extract sulfite ions into the organic phase. Subsequently, the organic phase is heated and subjected to stripping to yield concentrated sulfur dioxide gas therefrom.
Several problems characterize the Wiewiorowski process which make it less efficient than my process disclosed in U.S. Pat. No. 4,101,643. These were discussed in the referenced patent. Additionally, another disadvantage of the Wiewiorowski process is that some ammonia will be retained in the organic extract and released together with SO.sub.2 during the subsequent steam stripping. Separation of SO.sub.2 and NH.sub.3 in the regenerated gas stream is difficult.
Further, though it is not a major object to the ammonium phosphate scrubbing process, the use of ammonium salt of this type tends to cause the formation of an undesirable "plume" at the top of the flue gas stack--an obviously undesirable aspect in terms of real, as well as apparent, atmospheric pollution. The plume results from finely divided ammonium salts apparently formed in the gas phase reactions.
In another previously known SO.sub.2 removal process, the aqueous solution of sodium sulfite is used for scrubbing purposes. The pregnant scrubbing liquor, which is essentially an aqueous solution of sodium hydrosulfite, is then directly (without intervening processes) decomposed thermally to yield sulfur dioxide. In this method of proceeding, however, the heat input required to effect thermal decomposition of the hydrogen sulfite, as well as to vaporize a large quantity of water, is great. The water vapor thus emitted with sulfur dioxide must be subsequently condensed. The development of the large quantity of water vapor also complicates control of the decomposition process.
Another serious drawback of the described process involving direct decomposition of the sodium hydrosulfite is that at the temperature level at which decomposition of the aqueous hydrosulfite occurs, a portion of the hydrosulfite converts to sulfate. This conversion commonly yields from 5% to 10% sodium sulfate by-product, which represents a corresponding loss in the total sodium value required in the system for scrubbing purposes. In addition, removal of the sodium sulfate by-product is a complicated procedure involving several process steps.